I guess that's the heart of it and understand the tradeoffs noe more than ever. Having built a couple of well documented amps from schematics - push pull, etc. - the lack of components and direct-ness of the signal path speaks to me as elegant.
I understand the math as described but it brought up a question as I sift through my parts bin. Having never implemented a hum bias pot, I'm curious to know how I might incorporate the 10 Ohm / 2W pot I have on hand in lieu of the 50 Ohm pot in the schematic from post #16. Would a resistor from each filament to the cathode resistor/bypass cap get me close? For example, my 10 Ohm pot and some 20 Ohm resistors (in front of me) on each leg to the center terminal would get me 15 Ohms into the self bias network.
15 x 4.2 = 63 Ohms
800 + 63 = 863 Ohms
That seems low to me... but maybe I'm missing something and would need adjust the value of the cathode resistor if using the parts I have on hand.
You should buy 50ohm power pots , not very expensive ... sorry but you can't build a tube amplifier with the leftover parts in your drawer
Of course in the first place if you wanted that schematic , filament transformer(s) with center tap should have been on your shopping list ...
Of course in the first place if you wanted that schematic , filament transformer(s) with center tap should have been on your shopping list ...
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@Depanatoru totally! mostly here to learn something along the way. buying exactly as specified obviously - but also curious to know how different values effect the design and what is possible, recommended or frowned upon. re: the 50 Ohm pot… it is from the schematic in post 16 and as far as i can tell there are no filament center taps. the original post did have a CT on the filament secondary but now I’m leaning away from that schematic for that very reason although a virtual CT seems to be a pretty common approach.
50ohm is the minimum value for a filament pot , lower and the current is too high . If you add fixed resistors in series with the pot , the adjustment is limited and become pretty useless . Even if it is in the adjustment range we don't do things that way ... there is no need to improvise
Makes sense as most of them seem to be either 50 ohms or 100 ohms although I've come accross a few schematics recently that have lower value pots with series resistors on 2A3 filaments (see attached). Granted they're different circuits, but nonetheless, I'm curious to know why and more importantly how they work as implemented. I'm sure there's a good reason and probably missing something... Both schematics are from the Japanese magazine Tube Kingdom. The top is Namaoki and the bottom is Shishido.
Joneci,
From filament to filament . . .
1. One end of the filament to 20 Ohms to one end of the 10 Ohm Pot, the other end of the 10 Ohm pot to 20 Ohms then to the other end of the filament.
The pot wiper goes to the resistor bias network, where the 50 Ohm wiper would have connected.
That gives a range of 10/50 Ohms, 20% Range might be enough to balance out the hum, or it might not be enough to balance out the hum.
2. One end of the filament to 20 Ohms to the other 20 Ohms to the other end of the filament.
The junction of the two 20 Ohm resistor goes to where the 50 wiper would have connected.
This gives a very approximate hum balance.
Either 1. or 2. above gets your amplifier Up-And-Running!
Now, have fun listening!
Later, you can order a 50 Ohm Pot.
2.5V / 50 Ohms = 50 mA. The power from that is 0.125 Watt (1/8 Watt).
2.5A of 2A3 filament current Plus 0.050A more for the 50 Ohm pot will not hurt your power transformer's filament winding
From filament to filament . . .
1. One end of the filament to 20 Ohms to one end of the 10 Ohm Pot, the other end of the 10 Ohm pot to 20 Ohms then to the other end of the filament.
The pot wiper goes to the resistor bias network, where the 50 Ohm wiper would have connected.
That gives a range of 10/50 Ohms, 20% Range might be enough to balance out the hum, or it might not be enough to balance out the hum.
2. One end of the filament to 20 Ohms to the other 20 Ohms to the other end of the filament.
The junction of the two 20 Ohm resistor goes to where the 50 wiper would have connected.
This gives a very approximate hum balance.
Either 1. or 2. above gets your amplifier Up-And-Running!
Now, have fun listening!
Later, you can order a 50 Ohm Pot.
2.5V / 50 Ohms = 50 mA. The power from that is 0.125 Watt (1/8 Watt).
2.5A of 2A3 filament current Plus 0.050A more for the 50 Ohm pot will not hurt your power transformer's filament winding
Thanks for this! So for my understanding... the value of the variable resistor only determines the range for hum balancing, correct? Going with a higher value 50 Ohm or 100 Ohm would allow for more range - opportunity - to balance out the hum. So, lowering the value of the hum pot shouldn't effect the cathode resistor or bypass cap values?
joneci,
No.
The hum balance Range is how close the pot wiper can come to one end of the filament, and how far the pot wiper can come to the other end of the filament.
20 Ohms, 10 Ohm Pot, 20 Ohms:
2.5V filament.
The wiper's voltage adjustment Range is only 10 Ohms out of 50 Ohms.
That range is the central 20% of the 2.5V.
1V across 20 Ohms, 0.5V across 10 Ohms (at the center of 2.5V), 1V across the other 20 Ohms.
The wiper goes from 1V away from one end of the filament, to 1V away from the other end of the filament.
A pot that is across the whole 2.5V (no resistors on either side of the pot) gives the wiper 100% Range, whether the pot is 10 Ohms, 50 Ohms, or 200 Ohms.
The point of selecting the total resistance from one end of the filament to the other end of the filament is to provide a low enough resistance so that the product of the tube u (4.2) x (the total resistance across the filament/2) does not significantly raise the plate impedance, rp.
Of course, we do not pick too low of a resistance, or its current is too high.
Example: 2A3 filament is 2.5V @ 2.5 Amps; that is 1 Ohm when the filament is hot (it is much lower than that when cold; perhaps 1/4 Ohm).
So we would not use a 1 Ohm or 5 Ohm pot.
The 10 Ohm pot would draw 0.25A (1/4 Amp); most would consider that a little bit more current than they prefer.
2.5V x (0.25A)squared = 0.625 Watt. The pot should be rated for at least 3 Watts.
No.
The hum balance Range is how close the pot wiper can come to one end of the filament, and how far the pot wiper can come to the other end of the filament.
20 Ohms, 10 Ohm Pot, 20 Ohms:
2.5V filament.
The wiper's voltage adjustment Range is only 10 Ohms out of 50 Ohms.
That range is the central 20% of the 2.5V.
1V across 20 Ohms, 0.5V across 10 Ohms (at the center of 2.5V), 1V across the other 20 Ohms.
The wiper goes from 1V away from one end of the filament, to 1V away from the other end of the filament.
A pot that is across the whole 2.5V (no resistors on either side of the pot) gives the wiper 100% Range, whether the pot is 10 Ohms, 50 Ohms, or 200 Ohms.
The point of selecting the total resistance from one end of the filament to the other end of the filament is to provide a low enough resistance so that the product of the tube u (4.2) x (the total resistance across the filament/2) does not significantly raise the plate impedance, rp.
Of course, we do not pick too low of a resistance, or its current is too high.
Example: 2A3 filament is 2.5V @ 2.5 Amps; that is 1 Ohm when the filament is hot (it is much lower than that when cold; perhaps 1/4 Ohm).
So we would not use a 1 Ohm or 5 Ohm pot.
The 10 Ohm pot would draw 0.25A (1/4 Amp); most would consider that a little bit more current than they prefer.
2.5V x (0.25A)squared = 0.625 Watt. The pot should be rated for at least 3 Watts.
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i’m getting there. thanks for your patience.
so with no resistors on either side of the pot the range is the same no matter the value - 10, 20, 50 or 100 ohms. and if one was to go with a lower value for the hum pot, it would need to be rated higher because there would be more current.
that said, what is the function of the resistors on either side of the pot?
The resistors on either side of the pot has 3 effects:
Allows you to use a particular pot that you have.
Gives less total Range, works with some tubes, not with others (of the same type number, even with the same manufacturer, etc.)
Gives More Resolution 'of and to' the exact sweet spot of the hum cancellation.
Suppose you have four 20 Ohm resistors for each channel.
two 20 Ohm resistors in parallel, 10 Ohm pot, two 20 Ohm resistors in parallel.
That is 10, 10 pot, 10 Ohms in series. 10/30 = the new range of 33%.
Four more 20 Ohm resistors for the other channel.
See how that works?
Allows you to use a particular pot that you have.
Gives less total Range, works with some tubes, not with others (of the same type number, even with the same manufacturer, etc.)
Gives More Resolution 'of and to' the exact sweet spot of the hum cancellation.
Suppose you have four 20 Ohm resistors for each channel.
two 20 Ohm resistors in parallel, 10 Ohm pot, two 20 Ohm resistors in parallel.
That is 10, 10 pot, 10 Ohms in series. 10/30 = the new range of 33%.
Four more 20 Ohm resistors for the other channel.
See how that works?
Told you that 50ohm is the minimum normally used value , if you don't have a 50ohm pot you add resistors like in those schematics but the adjustment range will be limited .
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